Portable electronic devices such as IC cards, cameras, wristwatches, and those electronic devices that are not grounded, are prone to be erroneously operated or broken down due to the phenomenon of static electricity.
Such devices have heretofore been prevented from malfunctioning, either by erroneously operating or from breaking down, in the same manner as those electronic devices that can be usually grounded. Using a reference potential of a grounding circuit, however, the noise can be suppressed efficiently over a frequency region which is lower than that of the VHF band. Even if one point or more points are grounded, the grounding circuit is not quite effective to absorb the electromagnetic waves of large energy that are generated when the static electricity is discharged. Therefore, the noise must be suppressed in another manner.
Many examples to cope with this problem have been introduced in, for example, Electronics System, "Materials To Cope With Troubles Caused by Static Electricity" compiled by Norio Murasaki, entitled "Section 6, Countermeasures 5.6.2.3 Against Static Electricity in ECR, Electronic Circuit and Printed Board", pp. 222-224. However, many technicians are empirically aware of the fact that there is no simple method which offers sufficiently desirable effects. It has been attempted to absorb noise by extending the pattern of the ground side of the circuit board having the potential of the active side 2 or to increase the width of the pattern, or by a method taught in "Grounding of Housing", p. 84, "Practical Method of Decreasing Noise", Henry W. OTT., Bell Laboratory, U.S.A., translated by Takao Matsui. With the portable electronic devices which are not allowed to be grounded, however, noise is not emitted to the ground, and the concept of grounding cannot be pursued. In the electronic devices that cannot be grounded, a pattern of circuit board of a potential of the active side, that is formed to absorb noise, works as an antenna, and the structure connected to the potential of the active side of the metal housing to accomplish the grounding, works as an antenna. That is, with the pattern of circuit board that is not grounded or with the structure connected to the potential of the active side of the metal housing to accomplish the grounding, the electromagnetic waves are absorbed and are converted into eddy current to produce noise in the circuit network. Furthermore, under the condition where the electric charge is not discharged, the lines assuming the potential of the active side a of floated ground function to increase the difference of electric field distribution relative to the pull side. This becomes a cause of erroneous operation for field effect IC's.
As shown in FIG. 7, under such circumstances, three lines, i.e., an active side or power source line 2, an input line 4 and a pull side or ground line 3 establish an electrically important relationship. Described hereinbelow are the drawbacks that were not solved by the conventional art. For convenience of description, the high potential side of the power supply, whether positive or negative, is referred to as the active side or the power source line 2, and the low potential side is referred to as the pull side or the ground line 3.
FIG. 7 is a circuit diagram which schematically illustrates an input circuit. Here, a capacity or capacitance Ca exists between the active side 2 and the input line 4, and a capacity or capacitance Cp exists between the pull side 3 and the input line 4. The relationship Ca&gt;Cp will be described. It is obvious that electromagnetic waves generated by static electricity or the like are simultaneously superposed as an impulse on the active side 2 and on the pull side 3 of the power source voltage. Therefore, the potential appearing on the input line 4 forms a potential on the active side 2 via Ca. With a relationship Ca&gt;Cp being maintained, a potential on the pull side 3 also appears on Cp but a potential on the active side 2 appears on Ca. As a result, the potential on the input line 4 appears on Ca by an amount that corresponds to the difference between the potential on the pull side 3 and that on the active side 2. Accordingly, even if the switch SW is not turned on, the potential of the input line 4 produces a potential on the active side 2. Under this condition, the input circuit operates erroneously even when the switch SW is not operated. A resistance Rp is provided to form pull means, and a circuit 1 is provided in a stage which succeeds the switch SW.
FIG. 8 is a diagram of voltage waveforms that change with the lapse of time, and illustrates erroneous operations of the input circuit of FIG. 7 caused by the static electricity and the like, and wherein V.sub.1 denotes an active potential, V.sub.2 denotes a pull potential, V.sub.3 denotes a potential of the input line which is usually equal to V.sub.2, V.sub.T denotes a threshold potential of the circuit 1 under the ordinary condition which is equal to V.sub.2 /2, t denotes a time, t.sub.0 denotes a time at a moment when V.sub.0 which is the power source voltage is rapidly changed by the static electricity into V.sub.2 (t.sub.0), V.sub.2 (t.sub.0) denotes a potential at the time t.sub.0, V.sub.3 (t.sub.0) denotes a potential of the input line that is obtained by dividing V.sub.2 (t.sub.0) by the capacities Ca and Cp, i.e., V.sub.2 (t.sub.0).multidot.Cp/(Ca+Cp), V.sub.T (t.sub.0) denotes a threshold potential of the circuit 1, i.e., V.sub.2 (t.sub.0)/2, V.sub.2 (t) denotes a power source voltage which is given as a function of the time t, and its time constant is nearly equal to C.r where C denotes a resultant capacity consisting of capacities Ca, Cp, and a capacity of noise source, and r denotes an internal resistance of the power source, V.sub.3 (t) denotes a potential of the input line 4 given as a function of the time t, i.e., given by V.sub.3 (t).perspectiveto.V.sub.2 (t).multidot.Cp/(Ca+Cp) exp [-t/(CapRp)], and its time constant is nearly equal to Cap.Rp which is a product of the resultant capacity Cap consisting of the capacities Ca, Cp and the resistance Rp of the pull means (usually, Rp is considerably greater than r), and V.sub.T (t) denotes a threshold potential V.sub.2 (t)/2 of the circuit 1 given as a function of the time t.
The device will be described in more detail in conjunction with FIGS. 7 and 8. A man who is electrically charged may produce a voltage in excess of 10 Kv presenting a serious problem in handling electronic devices or portable electronic devices that are not permitted to be grounded. Here, if the potential V.sub.2 rapidly increases to V.sub.2 (t.sub.0) at the time t.sub.0 due to noise in the form of electromagnetic waves produced by the electric discharge, the threshold potential V.sub.T =V.sub.2 /2 of the circuit 1 changes into V.sub.T (t.sub.0)=V.sub.2 (t.sub.0)/2, and the potential V.sub.3 =V.sub.2 of the input line 4 changes into V.sub.2 (t.sub.0)=V.sub.2 +[V.sub.2 (t.sub.0)-V.sub.2 ].multidot.Cp/(Ca+Cp). If the capacities have a relation Ca&gt;Cp, the potential V.sub.3 (t.sub.0) of the input line 4 is apt to reach the threshold voltage V.sub.T (t.sub.0)=V.sub.2 (t.sub.0)/2 depending upon the magnitude of V.sub.2 (t.sub.0) and is changed by electromagnetic wave noise so that it becomes an erroneous input. For instance, if the capacities have a relation Ca= 1.5Cp, the potential V.sub.2 (t.sub.0) become more than 6 times as great as the potential under the ordinary condition of the potential V.sub.2. If Ca=4 Cp, V.sub.2 (t.sub.0) becomes more than 8/3 as great as V.sub.2, whereby the potential V.sub.3 (t.sub.0) of the input line 4 reaches the threshold voltage V.sub.T (t.sub.0)=V.sub.2 (t.sub.0)/2 to form an erroneous input.
The above way of thinking is supported by means for deriving a hypothesis that will be described below, and by experiment and investigation using practical electronic devices. Described below are the concept for improving the immunity against static electricity, means for deriving the hypothesis, and experimentation to prove the hypothesis.
(i) Concept
FIG. 9 is a schematic diagram related to energy of the electromagnetic waves and noise, wherein reference numeral 100 denotes a region which includes all of the electromagnetic waves such as VHF electromagnetic waves, infrared rays, visible rays, ultraviolet rays, and X-rays. The electric discharge is a phenomenon of an avalanche of electrons in which the electrons having a kinetic energy proportional to the voltage impinge upon an opposing metal electrode. Most of the electric charge migrates. The electric charge flows through paths having small impedances. Here, what is important is to construct the device so that the electric charge does not pass through the electronic circuit network. Upon impingement, the kinetic energy E is converted into electromagnetic waves. The kinetic energy E of an electron is given by, EQU E=1/2mv.sup.2 =eV
The energy is conserved and is converted into electromagnetic waves, i.e., EQU eV=h.nu., eV=hc/.lambda.
The wavelength .lambda. becomes a minimum at a point where the charged voltage V is a maximum, i.e., EQU .lambda.min=hc/eVmax
If ##EQU1## are substituted for the above equation, the wavelength .lambda. having a unit in the order of angstroms and the charged voltage having a unit in the order of Kv, there is obtained an equation, EQU .lambda.min[.ANG.]=12.4/Vmax[Kv]
If the charged voltage is known, therefore, the wavelength .lambda.min can be easily calculated. For example, if the charged voltage Vmax is 24.8 Kv, there are generated electromagnetic waves over a wide range, the minimum wavelength thereof being 0.5 angstrom. However, the energy is not all converted. The conversion efficiency .epsilon. is given by, ##EQU2## where, K: 1.1.times.10.sup.-9
Z: atomic number of the atoms that received impingement PA1 (a) The patterns and metallic structural parts on the circuit board connected to the active side 2 function as antennas depending upon the magnitude of energy of the static electricity, and absorb the electromagnetic waves to generate eddy currents which produce noise in the electronic circuit network. In particular, since the portable electronic devices cannot be ideally grounded, the noise energy must be consumed in the electronic circuit network thereby leading to erroneous operation (region indicated by 103 in FIG. 9). PA1 (b) The semiconductor in which silicon forms the base operates erroneously when it is irradiated with electromagnetic waves of a wavelength shorter than 10331.5 angstroms. PA1 (c) The device operates erroneously if the electric charge flows through the electronic circuit network, and the oxide film is broken (scattered) if the electric charge flows through the semiconductor. PA1 (d) The electronic circuit network is affected by the disturbance in the distribution of electric field in which the static electricity exists, when the electronic circuit network is provided with a field-effect IC which, in principle, is susceptible to the electric field.
I: discharge current
Therefore, part of the sparks of static electricity caused by the avalanche phenomenon of electrons is converted into electromagnetic waves which, however, is related to extranuclear electrons and have wavelengths longer than those of the X-rays. The range is indicated by a region 101 in FIG. 9.
With a semiconductor in which silicon forms the base, the electrons move freely when they are irradiated with electromagnetic waves or a beam of particles having energy greater than an energy gap Eg (=1.2 eV). In terms of the threshold wavelength, the electrons move freely when they are irradiated with electromagnetic waves having wavelengths shorter than 10331.5 angstroms, since .nu.=E/h, .lambda.=c/.nu.. The range is indicated by a region 102 in FIG. 9.
Thus, a metal part connected to the potential of the active side 2 serves as an antenna which absorbs electromagnetic waves at the time when the static electricity sparks in compliance with the aforementioned equation .lambda.min=12.4/Vmax. Therefore, eddy currents flow into the electronic circuit network to produce noise. In the electronic devices, the noise must be suppressed for all of the electromagnetic wave bands (region 104 in FIG. 9). In a grounded circuit, the noise can be suppressed by the single-point grounding for the electromagnetic waves of up to the VHF band that is referred to in a radio communications system, and can be suppressed by the many-point grounding for the electromagnetic waves of the UHF band (region where waveguides are handled), which, however, are not omnipotent. That is, interference to the electronic devices caused by static electricity must be eliminated by a method which does not rely upon the grounding circuit. The region based upon a new concept is indicated by 105 in FIG. 9.
Generally, electrically equivalent constants of a human body consist of a resistance of about one kiloohms and an electrostatic capacity of 200 pF to 500 pF. FIG. 10 shows electrostatic discharge characteristics for the amount of electric charge (Q) when the charged volatage (V) is 20 Kv, resistance (R) is 800 ohms, and electrostatic capacity (C) is 500 pF. The amount of electric charge (Q) is given by, ##EQU3##
If the side in which the discharge takes place has zero resistance, a discharge time .tau. given by, ##EQU4## is required until the electric charge of 10 .mu.C is discharged to 1/e. In practice, however, the side in which the discharge takes place has resistance to some extent, and a longer period of time is required for the discharge. If the electric charge passes through the CMOS IC in the electronic circuit network, the oxide film is completely destroyed (scattered). If the device is under an electrically charged condition in which the electric discharge does not take place, the potential of the active side 2 is distributed throughout the electronic circuit network that is constituted relying upon the idea of forming a grounding circuit. Therefore, erroneous input and erroneous operation caused by the electric charge take place on the pull side 3, in the input line 4 and in the field-effect IC.
Harmful influences to the electronic devices caused by the static electricity are summarized below.
(ii) Means for Deriving Hypotheses
A semiconductor integrated circuit (IC) chip has many input/output terminals (hereinafter referred to as IC pads). The layout of IC pads will be discussed below. Here, to simplify the description, the layout contains a minimum number of pads. FIG. 11 is a model diagram of the layout of IC pads that are mounted on a circuit board and susceptible to the static electricity. FIG. 13 is is also a model diagram of the layout of IC pads that are mounted on a circuit board and are susceptible to the static electricity. FIG. 12 is a matrix diagram for comparing the positional relations related to the IC pad layout of FIG. 11, and FIG. 14 is a matrix diagram for comparing the positional relations related to the IC pad layout of FIG. 13. The names of coordinates of the matrix are names of the functions of the IC pads. In FIG. 11, for example X.sub.IN and X.sub.OUT denote IC pads that constitute an oscillation circuit, and .phi..sub.1 and .phi..sub.2 denote IC pads that constitute a booster circuit. Symbol V.sub.DD denotes an IC pad which receives a potential of the active side 2, and V.sub.SS denotes an IC pad which receives a potential of the pull side 3. Symbol SW denotes an IC pad of the circuit 1 which is connected to an end of a switch in the input line 4. As for the pads X.sub.OUT and V.sub.SS located on either side of pad X.sub.IN in the model diagram of FIG. 11, symbol .circle. is put to the frames of X.sub.OUT and in the matrix diagram of FIG. 12 in the row of X.sub.IN and V.sub.SS. Next, as for X.sub.IN and SW on side of X.sub.OUT, symbol .circle. is put to the frames of X.sub.IN and SW in the row of X.sub.OUT. Symbols .circle. are put to the frames in the same manner as for .phi..sub.1, and then as for .phi..sub.2, SW, V.sub.DD and V.sub.SS, successively. Similarly, FIG. 14 is described based upon FIG. 13.
Next, a diagram of FIG. 15 for comparing matrixes is described by combining the upper right portion of the diagonal line of the matrix diagram of FIG. 12 which shows the pads that are mounted on the circuit board and are susceptible to the static electricity with the lower left portion of the diagonal line of the matrix diagram of FIG. 14. In FIG. 15, symbols .circle. that are located at symmetrical positions relative to the diagonal line are replaced by symbols . The matrix of symbols indicates that the layouts of IC pads of FIGS. 11 and 13 are in a neighborhood relationship to each other. That is, X.sub.IN and X.sub.OUT, .phi..sub.1 and .phi..sub.2, and V.sub.DD and SW indicate similar layouts having neighborhood relationship in FIGS. 11 and 13. A diagram of FIG. 20 for comparing matrixes is described by combining the upper right portion of the diagonal line of the matrix diagram of FIG. 17 corresponding to FIG. 16 which shows the layout of IC pads that are mounted on the circuit board and are confirmed to be little affected by the static electricity, with the lower left portion of the diagonal line of a matrix diagram of FIG. 19 corresponding to FIG. 18 which shows the layout of IC pads that are confirmed to be little affected by the static electricity after they have been mounted. In FIG. 20, symbols .circle. that are located at symmetrical positions relative to the diagonal line are replaced by symboles . The matrix of symbols indicates that the layouts of IC pads of FIGS. 16 and 18 are in a neighborhood relationship to each other. That is, X.sub.IN and X.sub.OUT, .phi..sub.1 and .phi..sub.2, and V.sub.SS and SW indicate similar layouts having a neighborhood relationship in FIGS. 15 and 17.
The IC that is easily affected by the static electricity after it has been mounted on the circuit board is compared below with the IC that is little affected, in regard to their similar points. ##EQU5##
If similar points common to the easily affected IC and the little affected IC are eliminated, since they are irrelevant to whether they are little affected or easily affected, the following fact becomes obvious. ##EQU6##
The results are discussed below. After mounted on the circuit board, the IC tends to be easily affected or little affected by the static electricity because of the reason that V.sub.DD (active side 2) has more patterns that stretch on the circuit board than V.sub.SS (pull side 3), since the circuit is designed based upon the idea of employing a grounding circuit. This fact is quite important to derive the following hypotheses.
Hypothesis 1:
The IC which is easily affected has V.sub.DD and SW (active side 2 and input line 4) that are adjacent to each other. That is, there exists a large stray capacity Ca between the active side 2 and the input line 4.
Hypothesis 2:
The IC which is little affected has V.sub.SS and SW (pull side 3 and input line 4) that are adjacent to each other. That is, a stray capacity Cp between the pull side 3 and the input line 4 is smaller than the above-mentioned stray capacity Ca.
(iii) Experiment to Confirm the Hypotheses:
The following two examples will be described to prove the hypotheses 1 and 2.
FIG. 21 is a diagram of the circuit of a digital wristwatch, in which reference numeral 4a denotes a switch pattern which forms an end of the input line 4, and 2b denotes a switch spring which assumes the potential of the active side 2. On the switch pattern 4a is disposed the switch spring 2b in an opposed manner over an area 9 of 10 mm.sup.2 maintaining a distance of 0.3 mm, to thereby form an air capacitor. Under this condition, the device is easily affected by the static electricity and operates erroneously for various control operations. If the shape of the switch spring 2b is so changed that it is opposed to the switch pattern over an area 10 instead of the area 9, then the device becomes little affected by the static electricity. This fact verifies the aforementioned hypothesis 1. As a follow-up experiment, a capacitor was inserted between the switch pattern 4a and the potential V.sub.SS of the pull side 3 under the condition of the opposed area 9, in order to examine the immunity of the device against the static electricity. It was found that the device exhibited immunity if the capacitor had a capacity greater than a given value.
FIG. 22 is a diagram showing another circuit of the electronic wristwatch. The device is little affected by the static electricity when a temporarily formed V.sub.DD pattern 11 is not provided on the V.sub.DD pattern 2b adjacent to the switch pattern 4a, the V.sub.DD pattern 2b assuming the potential of the active side 2. On the other hand, if the temporarily formed V.sub.DD pattern is located adjacent to the switch pattern 4a as shown in the drawing, the device becomes susceptible to the static electricity and operates erroneously such as displaying incorrect indication. This is one of the verifications related to the aforementioned hypothesis 2. The idea to lengthen the pattern of the active side 2 on the circuit board or to increase the area conforms to the way of thinking which is based upon the grounding circuit, but is not quite effective to suppress the noise that stems from the static electricity and rather makes the situation worse.
Even if the existing grounding circuit is employed in an attempt to cope with the static electicity, a discharge current of static electricity flows from the metal housing into the electronic circuit network, and the IC is broken down or electrical data in the circuit network are disturbed. Further, electromagnetic waves of large energy generated by lightning or the discharge of static electricity are absorbed by the metal housing and by the pattern of the active side 2 to form eddy currents which then generate noise to disturb the electrical data in the electronic circuit network. Moreover, when the electronic device is placed under a condition where the static electricity is not discharged, the active side 2 (ground side) that spreads throughout the circuit board works more effectively to receive the static electricity than the pull side 3. Therefore, the electric field acts more greatly upon the active side 2 than upon the pull side 3, and the device operates erroneously.